Gas turbine engines and particularly turbofan engines used in aircraft have a fan with a hub and plurality of fan blades disposed for rotation about a central axis. Catastrophic damage can occur to the aircraft and the occupants if a broken fan blade is propelled radially outwardly from the rotating hub under centrifugal force and impacts on the aircraft fuselage.
To prevent such damage, it is common to include a generally cylindrical fan case about the periphery of the fan for compliantly containing a broken fan blade. The fan case includes an elastic blade containment belt made of multiple plies of synthetic Kevlar (trademark) fabric for example. The compliant belt absorbs the impact from the broken blade and retains the broken blade within the fan case by destructive delamination and tearing of the Kevlar fabric under impact loading. Similar delaminating multiply fabrics are used in bulletproof body armour, fall arrest safety harnesses, etc.
Very high loads are imposed by the belt during impact on a belt support shell disposed inside the belt. The belt and belt support shell deflect under impact to an oval or elliptical shape. To avoid destructive contact between the interior of the deflected belt support shell and the remaining rotating fan blades, a degree of clearance is required and the belt support shell must be rigid enough to maintain sufficient clearance. The interior of the belt support shell may be lined with lightweight acoustic material and a wear plate to maintain the aerodynamic profile of the fan case and to reduce noise levels. On impact with the broken blade projectile, the acoustic material and wear plate shatter and disintegrate. Due to the displacement of acoustic material, the remaining clearance between the belt support shell (deflected to an oval shape under impact loading) and the rotating fan blades is sufficient to prevent destructive contact.
The design task for such belt support shells is substantial. The belt support shell must have a high strength to weight ratio with minimal profile, as must all aircraft structures to minimize aircraft lift requirements and drag losses. The belt support shell is disposed inside the compliant belt and therefore must allow the broken blade projectile to pass through the shell with as little resistance as possible to minimize the risk of contact between the remaining fan components and the broken blade or the deflected oval-shaped belt support shell. The extent of damage to the belt support shell on impact with the broken blade is highly unpredictable. Substantial safety factors must be provided to avoid catastrophic failure and inward collapse of the shell into the rotating fan. The remaining undamaged portions of the belt support shell must provide sufficient support to resist the loads imposed by the compliant belt as it stretches and delaminates on impact.
In general, conventional belt support shells are fabricated of aluminum for reduced weight, however, in some circumstances steel shells are also used. The broken fan blade passes through the thin walls of the belt support shell due to the high kinetic energy imparted by the high rotational velocity of the fan assembly. The blade projectile does not pass directly through the belt support shell in a purely radial direction, but rather cuts an arc of approximately 90.degree. through the belt support shell as the blade is expelled radially under centrifugal force while continuing to rotate rapidly about the engine axis.
As the blade projectile impacts the compliant belt, significant tensile forces are generated in the belt as kinetic energy from the blade is absorbed. These forces are resisted by the belt support shell in its post impact or damaged state.
Conventional belt support shells are thin walled bodies of rotation which depend in large part for their strength on the structural shell behavior. A closed hoop or shell has significantly greater structural strength than an open section. Shell structures often have poor structural resistance after impact due to the damage caused by the blade passing through them. In the damaged state, high post impact loads applied by the compliant belt have in some cases caused complete inward collapse of the belt support shell. Such collapse is catastrophic leading to engine failure and possibly explosion. Needless to say such performance is completely unacceptable and may represent a greater risk to aircraft safety than the broken fan blade projectile itself. As a result the belt support shells must be designed to resist the tensile loads from the compliant belt in their damaged state. This has led to relatively robust and heavy designs for belt support shells which detrimentally effects the performance of the engine and aircraft as a whole. The structural strength of a shell is dependent on a closed body which can transfer and distribute forces throughout the thin wall of the shell. However, when impact occurs with a projectile, damage to the shell compromises the shell strength significantly.
The modern development of fibre composite structures has not been introduced to a great extent into aircraft structures. Traditional use of aluminum and metal alloys remains prevalent despite the cost and weight savings possible through use of fibre composites. Introduction of new materials involves significant testing and regulatory approval. To date most applications of composite materials in aircraft has been to less critical elements exposed to low stresses, such as door panels.
Composite materials utilize high strength fibres disposed in a relatively brittle matrix. Although the strength to weight ratio is superior to many metal alloys, the ductility of metal alloys provides preferred levels of safety over relatively brittle composites when exposed to high impact loadings. Composite structures are easily fabricated into complex shapes. However, the acceptance of fibre composite structures has been very slow especially where passenger safety is a paramount concern, such as in aircraft and automobile design.
Examples of fibre composite structures and fabrication methods are provided in the following U.S. patents: U.S. Pat. No. 4,086,378 to Kam et al shows a cylindrical composite structure with helical, axial and circumferential reinforcing ribs forming an interior lattice; U.S. Pat. No. 4,012,549 to Slysh describes a high strength composite structure with an isogrid lattice of equilateral triangles; U.S. Pat. No. 4,137,354 to Mayes, Jr. et al describes a ribbed composite cylindrical structure and manufacturing process; and U.S. Pat. No. 4,973,521 describes a composite bonded structure applied to an aircraft fan blade.
It is an object of the invention to provide a fiber composite shell to reduce the weight and expense involved in the conventional use of aluminum blade support shells.
It is a further object of the invention to provide a fiber composite belt support shell which retains much of its structural strength after impact with a broken blade projectile.
In particular, it is an object of the invention to provide a belt support shell which presents no impediment to outward movement of a broken blade but, resists the high impact stresses imposed by the compliant containment belt.